I have been using some AA lithium rechargable batteries that i found on @#$%& from china and they work great. They are rated at 3000mWh in reality its about 2100mAh and i run the Impact on 5Khz plus full backlight and mostly use the speaker on full volume because i use Gen mode. They last 8 hours no problem.

As for the capability in which the voltage regulator can handle the maximum before frying it is in this reply from Dilek when i asked her if i can use 1,6volt NiZn AA batteries in the Impact .. in which their optimal charge reaches 1.9 volts so here is the reply:

" Ok here is the answer from the engineer:
''Hi,

NiZn rechargeable batteries are not tested with the product, but according to their characteristics; they are safe to use with Impact (the Impact battery circuitry is tested upto 10V’s). The voltage indicator bar must be disregarded (likely will have no function) because the battery range is optimized to cover alkaline and NiMH battery voltages (Such as AT Gold).

You may encounter some unusual shutdown events when your batteries are near empty, but these are temporary and will not damage the detector. ''

Personally, I would just get NiMH rechargeables.
They come with the pro pack and I know for sure that they work well on my racer 2 and relic.
I use eneloops mostly but I have many cells from different brands from when I was experimenting with my analyzer chargers. They hold a charge really well while sitting and give good life. Plus the regular ones (not black) can be recharged 2100 times and new nimh don't have a memory.
You can get the high capacity ones too, but they will discharge from sitting faster than the regular capacity ones.
Normal eneloops rate between 1950-2200 mAh out of all the 40+ cells I have of those.
As you can see from the picture I was a bit obsessed with testing different brands lol.

Yeah, my question is why? The ones that are recommended and supplied by the manufacturer work and last just fine? Some of you guys must have marathon detecting sessions! I assume you are looking for longer lasting batteries?

I'm using Eneloops in my Relic and my Racer came with the Akaline Varta batteries.... I've used Eneloops in all of my machines now, along with all the other NiMh's that I have. Not a single machine has lost one single battery bar, by the time I was wore out.. I bought one of those OPUS BT-C3100 V2.2 chargers, and it does a Fantastic job on all of my batteries....